1. Field of the Invention
The present invention relates to a planar light source device and a display device using the planar light source device.
2. Description of the Related Art
There are known two types of planar light source devices each of which is disposed at the back surface side of a liquid crystal display device. One type of these planar light source devices is called an edge light type, and the other type thereof is called a just-below type. According to the edge light type planar light source device, a light source is disposed at one side surface or at both side surfaces of a light guiding plate of the light source, and lights from the light sources are emitted from the end surfaces of the light guiding plate so that the light is emitted in a planar shape.
The edge light type planar light source device has such a feature that the thickness of the planar light source device can be reduced, however, it needs the light guiding plate having the same size as or larger than the display area as the light guiding plate and thus the weight of the planar light source device is increased.
On the other hand, according to the just-below type planar light source device, plural linear light sources (fluorescent tubes) are arranged just below the light emission face opposing a liquid crystal panel, and the just-blow type planar light source device needs no light guiding plate, so that the weight thereof is small. However, when the distance from the light source to the light emission face is small, unevenness in brightness (in general, light is bright just above a linear light source whereas light is dark between linear light sources) is easily visually identified, and thus it is necessary to increase the distance from the light source to the light emission face. Furthermore, when the distance between each light source and a metal housing disposed at the reflection face side so as to confront the light emission face is small, leak current occurs between the metal housing and each light source, so that the linear light sources are not normally turned on. Therefore, this type of planar light source device has such a feature that it is necessary to increase the distance between the metal housing and each light source, and the thickness of the planar light source device (in general, 3 mm or more) is increased.
Recently, the liquid crystal display device has been broadly used for not only a personal computer, but also a television set, and rapidly increased in size. In general, the former just-below type planar light source which provides high brightness and is light in weight has prevailed as a planar light source device used for a large-size liquid crystal display device. Therefore, it has been required to overcome the drawbacks in thickness, unevenness of brightness, etc. as quickly as possible with keeping the high brightness and the light weight which are advantages of the just-below type planar light source device. An illumination device in which an auxiliary light source (point light source) is provided just below a main light source (fluorescent tube) as disclosed in JP-A-2000-310776 (pp 3-4, FIG. 1) has been proposed as a means for overcoming the unevenness of brightness of the just-below type planar light source device. Furthermore, JP-A-2004-39482 has proposed a planar light source device in which rod-like light emitters each comprising a rod body and an LED are juxtaposed with one another.
According to the illumination device proposed in JP-A-2000-310776, it is possible to reduce the distance from the main light source (fluorescent tube) to the light emission face, however, it is impossible to reduce the distance between the main light source and the metal housing at the reflection face side. Furthermore, the auxiliary light source is equipped with a light guiding plate in addition to the main light source. Therefore, this device has a drawback that the construction is complicated and the thickness of the light guiding plate is increased. In addition, the fluorescent tube serving as the main light source contains mercury which is an environmental load material although the amount of mercury is minute, and thus it has been required to develop a planar light source device using a light source which takes the place of the fluorescent tube.
According to the planar light source device disclosed in JP-A-2004-39482, the LEDs are designed so that the illumination intensity of each LED is individually adjustable. Therefore, it is possible to adjust unevenness in brightness caused by dispersion of the LEDs, however, it is impossible to improve such unevenness in brightness along the longitudinal direction of the rod bodies that the brightness is higher in the neighborhood of each LED, but the brightness is lower as the position is far away from each LED. Particularly, since the LEDs are brought into close contact with the rod bodies, this type of planar light source device has a problem that light which travels in each rod body by total reflection and is a part of light emitted from each LED is reduced, and thus the amount of light emitted around each LED is increased.